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Oct. 8, 1946. H. MAGNUSKI 2,408,791 RADIO COMMUNICATION SYSTEM Filed June 21, 1945aI .wo“2532...5;.. 3, Sheets-Sheet 1 2» TS BL. 3.Ai4<IB:m]lu%2ras.d?‘g .3 , 532.8:% mun;_mhm ELa. U INVENTOR. HENRY MAGNUSK] .2HMREE . ATTORNEY Oct- 3, 1946- 2,408,791 H. MAGNUSK! RADIO COMMUNICATIGN SYSTEM Filed “June 21, 1943‘ s Sheets-Sheet 2 P5 1u5m2._3oz4 (1.8% 51W. v2. . 82:2: mlEHi4o1.: “,.97!3. __=2: . 28. 0 3 m3 . v.. .v 0 .053 3v 812.5%: o A. §v 3SQ.@28umm4:3m5:8 mokpqzimEowa INVENTOR HENRY MAGNUSKI 743444“ ATTORNEY Oct. 8,1946. H, MAGNUSKI I 2,408,791 RADIO COMMUNICATION SYSTEM Filed June 21, 1943 3 Sheets-Sheet 5 EGTIFIER 33 B REAGTANCE MODULATOR I3 > FIG.5 RELATNE DISCRMINATOR OUTPUT VOLTAGE 20 T IO \ I INVENTOR. -|o . . _ HENRY _ 2 o . l 2 a RELATIVE F- u VOLTAGE mvur m . MIGROVOLTS AT ANTENNA GROUND GIR- I9. 7 BY 4 , MAGNUSKI _ - W ,~ ATTORNEY , 2,408,791 Patented Oct. 8, 1946 UNITED STATES PATENT OFFICE RADIO COMMUNICATION SYSTEM Henry Magnuski, Chicago, 111., assignor to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application June 21, 1943, Serial N 0. 491,651 , . 9 Claims. 1 (Cl. 250-43) 2 . mediate frequency section of the receiving chan The present invention relates to improvements nel during signal reception. in radio communication apparatus and more par According to a further object of the invention, ticularly to improvements in combination radio the modulator stage of the transmission’ channel transmitter and receiver systems of the charac is controlled by a variable bias derived from the ter used. in police and military communication receiving channel in order automatically to con work, for example. trol the carrier frequency of asignal carrier tra Complete self-contained combination transmit versing the intermediate frequency section of the ter and receiver units, both of the portable and receiving channel during signal reception. I ?xed position types, are now extensively used in many forms of radio communication work and 10 It is another object of the invention to provide improved facilities whereby the system may be are'especially useful in two-way police and mili utilized for'signal transmission and reception at tary communication work. Such units must of the same carrier frequency without» altering the necessity be rugged, light in- weight, and easily tuning of any of the tunable stages of the system. manufactured in production quantities at low It is a further object of the invention to pro cost. They must also be capable of being easily vide an improved radio transmitter wherein two and rapidly conditioned to ‘operate either as a Oscillators arranged in tandem are utilized to pro transmitter or as a receiver, and should be eas duce for radiation a signal modulated carrier. ily tunable to’ transmit or receive at any desired According to a further object of the invention, carrier frequency within an allotted frequency band. Other requirements of a unit of this type 20 the transmission channel is provided with a mixer stage which follows the master carrier producing are that the unit have sufficient signal radiating oscillator and utilizes a crystal oscillator having power and sufficient sensitivity of reception to a resonant frequency equaling the‘ center inter permit high quality two-way communication to mediate frequency of the ?rst intermediate fre be held over substantial distances, and that the quency section of the receiving channel to obtain receiving channel of the unit be capable of main the desired frequency of carrier radiation. taining its sensitivity. in receiving a signal car It is a still further object of the invention to rier having a drifting carrier frequency. provide an improved arrangement for selectively In general, it is an object of the present inven tion to provide an : improved combination radio transmitting and receiving system which meets I all of the ‘requirements outlined above in a high l'y satisfactory manner. It is another object of the invention to provide a system of the frequency modulated type which meets all of the requirements outlined above. It is a further object of the invention to provide a combined transmitting and receiving system in which porticnsof both the transmission and. re ceiving channels are used both during signal transmission and signal reception, thereby to minimize the number of component parts of the system without sacri?cing. desirable operating features. ' ' . According to another object of the. invention, a combination frequency modulated transmitting and receiving'syste'mi is provided which'includes ' rendering the transmission and receiving chan nels active and inactive in a manner such that all of the facilities mentioned above are selectively, automatically and appropriately rendered active and inactive as the two channels are selectively conditioned for signal transmission and signal re ception. The invention, both as to its organization and method of operation, together with further ob jects and advantages thereof,- will best be under stood by reference to the following speci?cation taken in connection with the accompanying drawings, in which: Figs. 1 and 2, when laid end to end in the order named, illustrate a combined frequency modu lated radio transmitting and receiving system characterized by the features of the invention briefly referred to above; - _ a receiving channel of the superheterodyne type, and in which. improved facilities are provided for utilizing one of the oscillators of the transmis Fig. 3 diagrammatically illustrates the circuit arrangement of the cathode heaters of the elec tron discharge tubes included in the system sion channel as a local‘ oscillator'at a mixer stage shown in Figs, 1 and 2; v - Fig. 4 is a graph illustrating the noise and sig of the receiving channel during signal reception. nal response characteristics of the receiver; and In accordance with a further object of them Fig. 5 is a circuit diagram illustrating a modi vention,v improved facilities are provided for uti ?cation of the receiving equipment forming a lizing a portion of the transmission channel to obtain automatic frequency control in the inter 55 part of the system'shown in, Figs. 1 and 2. 2,408,791 4 Referring now more particularlypto Figs. 1 and 2 of the drawings, there is illustrated, partially in schematic form, a combination frequency mod ulated radio transmission and receiving system inductance element 42 and a phase shifting net work which includes the condenser 3'|a and the resistor 311). A suitable grid condenser 44 shunted by a grid leak resistor 43 is serially included in the which is well adapted for use as a complete port Cl input circuit of the oscillator tube M for the pur able unit and includes a transmitting section Ii) pose of maintaining the control grid of this tube and a receiving section II commonly coupled to at the proper operating potential with respect to an antenna ground circuit l9 through a tunable the cathode of the tube. antenna circuit I8. Brie?y considered, the trans The signal modulated carrier voltage developed mitting section | 0 comprises a combination auto 10 across the tunable frequency determining circuit matic frequency control and reactance modulator stage I3, a tunable master oscillator M, a tunable frequency doubler network IS, a tunable trans 4| is impressed between the input electrodes of a tube 41 included in the frequency doubler |5 through a network which comprises the coupling mitter mixer I6, a power ampli?er l1, and the condenser 49 and a resistor 48. This tube is pro tunable antenna circuit Hi, connected in tandem 15 vided at its output side with a tunable frequency in the order named. The receiving section II of selective circuit 52, which includes a ?xed in the system comprises the tunable antenna cir ductance element 52a shunted by a ?xed tuning cuit I8, a tunable radio frequency ampli?er 20, condenser 52b and an adjustable tuning con a ?rst mixer or converter stage 2|, a ?rst inter denser 520, and is tuned to a center frequency mediate frequency ampli?er 22, a second mixer 20 substantially twice the center resonant frequency or converter stage 23, a second intermediate fre~ of the frequency determining network 4| forming quency ampli?er 24, a ?rst limiter 25, a second a part of the master oscillator stage l4. Anode limiter 26, a frequency discriminator 21, an audio current is supplied to the tube 4‘! through a ?lter frequency ampli?er 28, and a loud speaker 29, all resistor 54 and the inductance element 52a, and connected in cascade in the order named. As 25 the usual direct current isolating condenser 5211 pointed out below the system may be selectively is provided in the circuit 52 to isolate the anode current path from ground. In accordance with the present invention, the set to operate as a receiver. In order to render carrier voltage developed through operation of the audio section of the receiving channel H in 30 the tunable master oscillator l4 and the tunable operative to pass noise signals appearing in this frequency doubler I5 is utilized as a heterodyning channel during intervals when a desired signal frequency source for converting a received fre is not being received, muting or squelch apparatus quency modulated radio frequency carrier into a is provided which comprises a high pass ?lter correspondingly modulated intermediate fre controlled to operate either as a transmitter or a receiver and, when conditioned for operation, is network 33 coupled to the output side of the fre quency discriminator 21, a noise ampli?er and recti?er 3|, a direct current ampli?er 32 and a muting oscillator and recti?er section 33. These quency carrier in the ?rst mixer stage 2| of the receiver channel ll. When, therefore, it is de sired to utilize the system to transmit and re ceive signals at a ?xed and preestablished carrier stages are connected in tandem in the order frequency without altering the tuning of the named, and respond to noise voltages appearing tunable circuits in the system incident to a change from transmission to reception, or vice versa, it is necessary to increase or decrease the in the receiver channel to impress a blocking bias voltage upon the audio ampli?er 28 in the manner c explained below. More speci?cally considered, the transmitting section of the system comprises a microphone |2 which is arranged to impress audio frequency voltages developed during operation thereof be tween the input electrodes of the combination frequency control and modulator tube 38 through a. coupling network which comprises a microphone transformer 35, a condenser 36, and a resistor 31. The space current path through the tube 38 is connected in shunt with the space current path through the tube 40 of the master oscillator l4, and also shunts the tunable frequency deter mining circuit 4| of the master oscillator M. This oscillator is of the conventional tuned plate cir cuit type, the frequency determining circuit 4| thereof comprising a, ?xed inductance element 4|a which is tuned to the desired resonant fre- _ quency by means of the shunt connected ?xed ‘condenser MI) and an adjustable tuning con denser 4|c, Operating potentials are supplied to the anodes of the tubes 40 and 38 through a re sistor 46 and the inductance element 4|, a low impedance direct current blocking condenser 4|d being provided in the tunable circuit 4| in order to isolate this direct current path from ground. The tunable frequency determining circuit 4| of the ‘oscillator I4 is regeneratively coupled to the input electrodes of the tube 40 by means of an inductance element 42 which is inductively coupled to the inductance element 4|. It is also coupled to the input electrodes of the frequency control and modulator tube 38 by means of the output frequency of the frequency doubler |5 by an amount equal to the value of the intermediate frequency utilized in the ?rst intermediate fre quency ampli?er section 22 of the receiver chan nel ||. Whether or not the output frequency of the doubler I5 is raised or lowered to provide the desired frequency of carrier transmission, will of course depend upon whether the doubler output frequency is above or below the particular car rier frequency at which transmission is to be ef fected. In the particular arrangement illustrat ed, a piezoelectric crystal 58 having a resonant frequency equal to the intermediate frequency utilized in the ?rst intermediate ampli?er 22 of the receiver is utilized to increase the frequency of a transmitted signal carrier above the carrier frequency appearing across the frequency selec tive circuit 52 by an amount equal to the inter mediate frequency utilized in the ?rst intermedi ate frequency ampli?er 22. More speci?cally considered, the tunable circuit 52 is coupled to the input electrodes of the transmitter mixer . tube 63 through the shunt connected crystal 58 and condenser 59.‘ A grid leak and condenser network comprising the two resistors 55 and 56 ‘and a condenser 51 is provided for maintaining the proper bias potential between the input elec trodes of the mixer tube 63. For the purpose of driving the crystals 58 to maintain oscillation of the crystal at its resonant frequency, a tuned cir cuit 60 is provided which is suitably designed to resonate at the same frequency as the crystal 58 and comprises a ?xed condenser 60a shunted by 2,408,791 6 an adjustable inductance element 691). This net work is included in the screen electrode circuit ‘of the mixer tube 63‘ and also in the path compris ing the resistor 62 over which, the required. oper ating potential is positively applied to the screen electrode of the tube 63. . The mixer tube 63 is provided with a tunable frequency selective output circuit 64, which com prises a ?xed inductance element 64a shunted ‘by and- screen electrodes. of the tube. This crystal ‘has a resonant frequency of 6.815 megacycles which is greater than the intermediate frequency utilized inthe ?rst intermediate frequency am pli?er ZZ-by'an amount equal to the intermediate frequency utilized in the second intermediatefre quency ampli?er 23; A suitable biasing network comprising the series connected resistors 18a and 18b shunted by thegrid condenser 19 is provided ' the ?xed condenser 64b and the adjustable tun 10 between the input electrodes of the tube ‘Hiv for maintaining, the ‘proper bias voltage between these ing condenser 640 through the low impedance electrodes. direct current isolating condensers 64d and- Me. Noise and signal voltages appearing at the out This circuit is normally‘ maintained tuned to a put side of the second. limiter 2B are introduced frequency'which is equal to twice the output fre quency of the oscillator I4 plus the resonant fre—. 15 into the frequency discriminator 21. Brie?y con sidered, this discriminator comprises a tuned quency of the crystal 58‘, which latter frequency circuit 8-1,. a pair of diode recti?er tubes ‘88 and equals the ?rst intermediate frequency used in 89, the space current. paths of which are re the receiver channel I l-. The output voltage ap spectively shunted by load resistors 90 and 9|, pearing across the circuit. 64 is impressed across the input circuit of the power ampli?er I] 20' a radio frequency‘ by-pass condenser 93 having substantially negligible‘ impedance to frequencies through a coupling network which includes the ' condenser 65. , 7 of the order of. the second intermediate frequency, and a stabilizing condenser 92. More speci?cally, the resonant circuit 81 serves to tune the‘ fre stage 2| is illustrated as being resistance-capaci 25 quency discriminator network to a center fre quency equal to‘ the second intermediate fre tance coupled to the output circuit of the tunable quency and comprises a pair of series connected radio frequency ampli?er 20 through a network condensers 81b and 810 which are shunted by which includes the coupling condenser 68 and re an adjustable inductance element 81a. Prefer‘ sister 69. As indicated above, when the system is conditioned for reception, the tunable master 30 ably the last mentioned element is of the variable permeability type being provided. with an adj ust oscillator l4 and the tunable frequency doubler able powdered ferrous metal core, the position of I5 are utilized as a heterodyning frequency source which may be changed. to alter the inductance required to effect the desired carrier frequency of the element within the desired limits; The conversion in the ?rst mixer stage 2|; To this end, an inductance element 53 which is induc 35 circuit constants of the resonant circuit '81 are so chosen that the discriminator network is pro tively coupled to the inductance element 52a of vided with a band pass characteristic such that the frequency selecting circuit 52 is included in all desired signal components of a‘. frequency mod the cathode-ground circuit of the mixer tube 19. ulated carrier appearing in the second interme _ The output electrodes of this tube: are coupled to a ‘?xed tuned frequency selecting circuit 1 IV which‘ 4.0 diate frequency channel 24, 25, 26 may be de tected and impressed upon the input circuit of comprises a ?xed condenser 'Hb shunted by an’ the audio» ampli?er 28. The voltage appearing adjustable inductance element ‘Ha andis tuned across the. output side of the second limiter 26 is to the desired ?rst intermediate frequency of 4.3 impressed upon the discriminator network 21 megacycles, for example. Anode potential is through a coupling condenser'86 which is con supplied to the tube 10 over a path which in nected' at one. side thereof to the junction point cludes the inductance element 'Hu' and a. ?lter between the two condensers 81b and 81c. Audio resistor ‘l3 which is shunted by a by-pass ‘con frequency voltages detected through operation of denser 12. The tuned output circuit ‘H of the the discriminator 21' appear across the condenser tube 19 is coupled to the input electrodes» of the 9:3: and are impressed: upon the input side of the ?rst tube in the intermediate frequency ampli?er audio. frequency ampli?er 28 through a coupling 22. through a network which comprises the cou circuit which includes radio frequency decoupling pling condenser 14 and resistor 15. resistor 94, an audio frequency ?lter comprising The output side of the ?rst intermediate‘ fre the resistor 95 and condenser 96', an audio fre quency‘ ampli?er 22 is coupled to the input elec quencycoupling condenser 91, and a volume con trodes of the mixer tube 16 provided at the sec trol voltage dividing network comprising the two ond mixer or converter stage 23 in an obvious resistors '99 and H10 and a direct current block manner. This tube is provided with output elec ing condenser HH. It will be understood in this trodes which are bridged by a frequency selec regard that, the proportion of the available audio tive circuit 8'! tuned to the second intermediate frequency of 2.515 megacycles, for example, and _ 00 frequency voltage appearing across the series con nected; resistors 98* and I90» which is impressed comprising a condenser 81b shunted by an ad- ‘ upon the input circut of the audio frequency am justable inductance element 8H1. Anode poten pli?er 28,>is determined by the setting of the wiper tial is supplied to the tube 16 over a path which 99 'aiong the‘ resistor 98. ' includes the inductance element 8m and a ?lter As will be explained more fully below, noise sig resistor 83 shunted by a by-pass condenser 82. nal's. appearing in the signal transmission chan The voltage appearing across the frequency'se nel of the receiver in the absence of a received lective circuit 8| is impressed across the input signal modulatedcarrier are passed through the side of the second intermediate frequency ampli Referring now more in detail to the signal re ceiving channel Hv of the system, the ?rst mixer ?er 24 through a network which comprises the a coupling condenser 84v and a resistor 85. For the '‘ purpose of effecting the required carrier fre quency conversion at the second mixer stage‘v 23, the miXer tube 19 is provided with an oscillator section of the Pierce type which includesa piezo-v electric crystal 71 connected. between the control discriminator 21- and appear as detected audio voltages across the condenser 93'. Such detected voltages are impressed across the high pass ?lter network 30, and. those components thereof‘ hav - ing. frequencies» above the cutoff frequency of the ?lter network‘ are impressed between the input electrodes of the tube [96 included in noise ann 2,408,791 7 8 pli?er and recti?er 3I. More speci?cally consid ered, the high pass ?lter 30 comprises avpair of series condensers I01 and I02 and a pair of diode 88, are of the ?lamentary cathode type. The diode recti?er 88 must of necessity be of the indirectly heated cathode type since the cathode thereof is,vduring operation of the discriminator 2‘I, maintained at potentials substantially above the reference ground potential present upon the shunt resistors I03 and I00, and is designed to pass those components of noise voltages which have frequencies above the normal signal re producing band of the receiver. The noise am pli?er section of the tube I00 works into a noise recti?er circuit which comprises the diode sec tion of the tube and a. load resistor IIO. ?lamentary cathodes of the remaining tubes pro vided in the system. More speci?cally, the circuit arrangement of the cathodes provided in the var This 10 ious electron discharge tubes referred to above and also provided in the diagrammatically illus recti?er circuit is coupled to the anode of the tube I06 through a coupling condenser I08 which trated sections of the system, is shown in Fig. 3 is of appropriate impedance to pass any noise cur of the drawings. In this circuit, reference char rents which may be transmitted through the high acters corresponding to those used in Figs. 1 and pass ?lter 30. Anode and screen potentials are 15 . 2, but having the differentiating subscripts a and supplied to the tube I06 through the resistors I01 b, are used to identify the relationship between and I09, the second of which is by-passed to the cathodes and the respective associated cir ground through a condenser‘ III‘. cuit sections, as shown in Figs. 1 and 2. From Recti?ed noise voltages appearing across the a consideration of the circuit arrangement shown load resistor IIO are utilized to control the bias 20 in Fig. 3, it will be noted that the various cath between the input electrodes of the tube I I3 pro odes are effectively isolated at radio and audio vided in the direct current ampli?er 32. Theini frequencies by means of the separating ?lter net tial or threshold bias established between the elec works comprising the illustrated high impedance trodes of this tube is derived from a voltage divid choke coils and the low impedance by-pass con ing network, which comprises the series connected 25 densers. It will also be noted that current for resistors IIBa, H61), and H00 bridged across the energizing the various cathodes in the series available source of anode potential, and is pro parallel circuit is supplied by a direct current vided with a tap II‘I adjustable along the resistor source I35 through the contacts of a manually I IE to impress a variable positive potential upon operable “on” and “off” switch I36. The cath the control electrode of the tube I I3 through the odes IBa and Ho of the electron discharge tubes ?lter resistor H5. The biasing circuits connected respectively provided in the mixer I6 and the between the input electrodes of the tube II3 are power ampli?er H are arranged to be energized by-passed for audio frequency currents by means in series with each other and with a suitable cur of a condenser I I4. Screen and anode potentials rent limiting resistor I31 through the contacts of are applied to the ampli?er tube I I3 through the 35 a manually operable “press-to-talk” switch I38. resistor H8 and the resistors H6 and I I9, in This switch is normally spring biased to its open series, respectively. circuit position and may be utilized in the man The direct current ampli?er 32 as controlled ner explained below selectively to condition the by the variable bias voltage derived from the system for signal transmission or signal reception, load resistor I I 0, is utilized to control the starting 40 as desired. It is provided with a pair of normally and stopping of the muting oscillator and recti?er open contacts I38a which are closed to connect 33. This stage of the muting or squelch appa the microphone I2 across the primary winding ratus comprises a dual purpose tube I20 having of the transformer 35 only when the switch is an oscillator section which includes a tuned fre operated to condition the system for signal trans quency determining circuit I2I connected be » mission. From an inspection of the cathode cir tween the output electrodes of the tube through cuit arrangement, it will be apparent that this a by-pass condenser I22. The resonant circuit circuit has been carefully arranged to utilize the 'I2I is ?xed tuned to a particular frequency of voltage drops across certain of the cathodes as from 200 to 300‘ kilocycles and comprises an in bias voltages between the input electrodes of cer ductance element I2 Ia shunted by a tuning con tain of the other tubes provided in the system. denser I2 I12. It is regeneratively coupled to the For example, the voltage drop appearing across input electrodes of the tube I20 by means of a the cathode 25a of the tube provided in the ?rst feed back circuit which comprises an inductance limiter stage 25 is impressed between the ?la element I23 inductively coupled to the inductance mentary cathode I5a of the tube 4'‘! in the proper element I2 Ia and connected in series with ‘a par 55 direction to bias this cathode positively with re allel connected grid leak resistor I24 and con spect to the control grid of the tube. These bias denser I25 between the control grid and cathode voltages, as derived from the circuit network of the tube I20. Anode potential is supplied to shown in Fig. 3, are appropriately indicated in the tube I20 over a path which includesthe in Figs. 1 and 2 of the drawings by the illustrated ductance element I2Ia and a resistor I26. The battery symbols, and the relationship between the oscillator section of the tube I20 is coupled to the respective battery symbols and the voltage drops rectifying circuit of the tube through a coupling across certain of the cathodes shown in Fig. 3 condenser I29, and the indicated rectifying cir will be readily apparent from a careful compari cuit serially includes the diode recti?er section of son of the circuit shown in Fig. 3 with that shown the tube and the resistors I28, I32 and 102;. Any 55 in Figs. 1 and 2. bias voltage appearing across the load resistors Preferably, the transmitter mixer tube 03 is I28, I32 and 'I8b'during operation of the oscil a pentode of the well known commercial 3A4. lator and recti?er stage 33 is negatively applied type, the frequency doubler tube 4'! and the mas to the control grid of the ?rst tube in the audio ter oscillator tube 40 are commercial type 1T4 frequency ampli?er 28 over a path which com 70 pentodes, the automatic frequency control and prises the resistor I21, the resistor I00 and the reactance modulator tube 38 and the ?rst mixer lower portion‘of the resistor 98. tube ‘I0 are commercial type 1L4 pentodes, the In order to insure that the system will operate second mixer tube ‘I5 is a commercial type 1R5 efficiently with low battery current‘ drain, all of pentagrid converter, the two diodes 80 and 89 the tubes, with the exception of the discriminator 75 are of the commercial type 1A3 and 155, respec 2,408,791 10 9 plied between the input electrodes of the tube 38 effectively serves to_ vary the tuning of the net tively, the noise amplifier, and recti?er tube I05 and the muting oscillator and recti?er tube I20 work 4| in like manner, whereby the carrier out put of the oscillator I4 is reactance modulated in accordance with the audio signal voltage im pressed between the‘ input electrodes of the tube 38.’ This modulated carrier‘ voltage is impressed between the vinput electrodes of the tube 41- of the frequency doubler I5- through the coupling muting oscillator and recti?er tube I20 is con 10 condenser 49. Due to the action of the tube 41 in distorting the signal modulated carrier voltage trolled in the manner more fully explained below and- the action of the tunable frequency selecting to effect the desired starting and stopping of the circuit '52 in selecting only signal modulated car oscillator section of this tube. It will be under rier components having twice the frequency of ‘stood that the tuning elements of the various the carrier voltage developed at the output side tunable circuits provided in the system are gang of‘ the oscillator I 4, the modulated carrier voltage controlled to be operated in unison, so that fre appearing across the output circuit of the-dou quency alignment between the various resonant bler I5 has a ‘carrier frequency which is twice frequencies thereof is maintained during each that of the oscillator carrier output frequency, tuning operation. More speci?cally, the tuning element of the antenna circuit I8‘, the tuning ele 20 1. e. 40 megacycles in the case assumed above. The signal modulated carrier voltage appearing ment of the radio frequency ampli?er 20, and the across; the frequency selecting circuit 52 is im adjustable condensers 64c, 52c and Mo, respec pressed‘ between the input electrodes of the trans tivelyrprovided at the tunable stages I6‘, i5 and are commercial type 135 pentodes, and the direct current ampli?er tube H3 is a commercial type 1L4 pentode. Suitable screen potentials are ap plied to the tubes 63, 4'1, 40', 38 and 16 over direct current paths which respectively include the ?lter resistors 52, 50, 45, 39 and 80, respectively. The potential applied to the screen electrode of the mitter mixer tube 63 over a path which includes I4, are mechanically connected in the manner in , the coupling condenser 59 and the heterodyning dicated by the dash line U, so that all of the piezo-electric crystal 58.» As previously explained, enumerated tuning elements may be operated in this crystal has a» resonant frequency which is unison. equal to the ?rst intermediate frequency used Brie?y to consider the operation of the sys inv the‘ receiving channel II of the system. Ac tem, it will be understood that when the switch I36 is operated to its closed circuit position, the 30' cordingly, this. crystal, acting in conjunction with the tuned circuit 60,. functions to produce a car cathodes of all tubes provided in the system, with rier voltage which is electronically mixed in the the exception of the cathodes I50: and I-"Ea of the tube 63' with the carrier frequencyoutput across tubes provided in the mixer ‘I6 and the power am the tuned circuit 52, so that a carrier is produced pli?er H, are energized from the current source I35. at the output side of the mixer tube 53 having. a If now the push-to-talk switch I33 is oper frequency equal to twice the output frequency of the‘. oscillator I4 plus the ?rst. intermediate frequency. This carrier voltage is. frequency modulated in accordance with the audio fre quency voltage applied to. the. input electrodes of the modulator tube 38. At theioutput side of the‘ ated to its closed circuit position, the cathodes 16a, and I-‘Ta are also energized. Duevv to the ?la mentary character of the energized cathodes, they‘ are rapidly heated‘ to electron emitting tem peratures following the energization thereof. When the two switches I35 and I38 are thus operated, the system is conditioned for signal transmission at the particular carrier frequency established by the tuning of the ?ve tunable stages I4, l5, l6‘, i1 and I181 of the transmission channel. In this regard it will be understood that when space current flow through the: tube 45' is initiated; the master oscillator I4starts to oscil late ‘at a carrier frequen'cy"which is primarily determined byv the " setting of the tuning con tube 63,. this particular signal modulated carrier voltage is selected throughthe action of the tuned frequency selecting‘ circuit 64 and is impressed 45 across. the‘ input circuit of the power amplifier I‘! through the coupling condenser 65. After being ampli?ed by the ampli?er I1, the voltage is trans mitted through the tunable antenna circuit I8 50: and impressed across the antenna ground circuit t9 for radiation. ,Referring now‘ more speci?cally to the‘ func-> tion performed by the tunable transmitter mixer I6, it is pointed out abovethat the tunable fre quency selecting circuit ~64 is tuned to respond denser 4'I:c. and‘. is. secondarily determined by the magnitude of the bias voltage between the control grid and cathode of the tube '38. More generallyv considered, if the receiving channel II of the system is designed to operate 'with a first. inter- 55' only to a signal. modulated carrier having a car rier frequency which is greater than twice the mediate- frequency of" 4:3 megacyclesl and. signals output carrier frequency of the oscillator I4-by are‘to be transmitted and received’ at a. carrier van amount equal to the intermediate frequency frequency: of‘ 44.3 megacycles, the condenser 4'I‘c utilized inv the ?rst intermediate frequency chan is so‘ adjusted that with zero bias upon the con nel 22,0f the receiver. Since the carrier voltage trol grid of vthe tube: 38, the masterv oscillator I41 appearing across- the’ tuned output circuit 52 of will produce a carrier voltagehaving a frequency the frequency doubler‘ I5 is used as a heterodyn of" 20 megacycles and the other’ tunable. circuits ing‘ frequency source at‘ the'?rst mixer stage 2i of the transmission- channel. I'Il‘are adjusted'am during reception and this frequency is mixed cordingly. With the frequency of the signal‘ car I with the; frequency produced by the crystal 58 to‘ rier thus determined‘, an audio frequency voltage produce‘ at frequency of’ carrier radiation which is developed: through operation of‘- the microphone equal to the sum of the‘ two frequencies, signal I2 is impressed‘ through the microphonegtrans reception and‘ transmission may be held at the former >3 5' and the coupling condenser 36' between same carrier frequency. Thus if the crystal 58 the. control grid and cathode of the modulator tube 38. The resulting audio frequency variation 4 has a’ resonant frequency of 4.3 megacycles, equal ing the center intermediate frequency used in the of the voltage between thecontrol grid and cath ?rst intermediate frequency‘ section of the re ode of the tube 38'. effectively changes thereac ceiver' channel I I", and the tuning. elements of the tance of the tunable. frequency determining cir tunable stages I4, I5, I6, I'I', I8’ and 25 are ad cuitJ4‘I' of'the master oscillator , I‘4' at a correspond. ing rate.v In other words‘, varying the voltage ap 75 justed by means of the‘ adjusting element U to' 11 12 a setting wherein the carrier output frequency of the oscillator I4 is 20 megacycles and the car rier output frequency of the doubler I5 is 40 megacycles, then the tunable stages ll, [8 and 6‘! to the input side of the tunable radio fre quency ampli?er 2D. This voltage, as ampli?ed by 29 are tuned to a carrier frequency of 44.3 mega cycles. This of course means that if the tunable put voltage is impressed between the cathode and stages of two remotely located sets of the char including the inductance element 53. It is thus acter illustrated are tuned for transmission and reception at the same carrier frequency, it is converted into a signal modulated intermediate As indicated above, the desired increase in the frequency of the radiated carrier over the carrier age and of the desired second intermediate fre quency, appears across the tuned output circuit the ampli?er 20, is mixed with the carrier output voltage of the frequency doubler l5, which out control grid of the tube 10 over a coupling path frequency carrier which is ampli?ed through the unnecessary to alter the settings of the tuning 10 ?rst intermediate ampli?er 22 and impressedbe elements of either set when the direction of trans tween the input electrodes of the tube 16 pro mission between the two systems is changed. vided in the secondmixer stage 23. Thus, the systems of the two sets may rapidly In the second mixer stage, the intermediate be altered for transmission in either direction frequency carrier output from the ampli?er 22 is - with a minimum number of manual operations 15 mixed with the carrier frequency produced on the part of the persons using the respective through operation‘ of the crystal Tl so that a beat sets for two-way communication. frequency carrier, modulated with the signal volt frequency appearing at the output side of the 20 8|. This modulated carrier, as selected through frequency doubler I5 is provided through the the action of the tuned circuit 8|, is transmitted action of the piezoelectric crystal 58. In con through the condenser 84 to the second interme sidering the manner in which this crystal is driven diate frequency ampli?er 24 where it is ampli?ed at its resonant frequency, it is pointed out that and transmitted successively through the limiter at this resonant frequency, the upper terminal 25 stages 25 and 26 to the input side of the discrim of the tuned circuit 52 is e?ectively at ground inator 21. In this discriminator the modulation potential due to the low impedance of this cir components of the second intermediate frequency cuit at the particular frequency in question. carrier, as represented by deviations in the car The resonant circuit 39 which is coupled between rier frequency from the, established center fre the cathode and screen electrode of the tube 63 30 quency, are detected in the manner pointed out is precisely tuned to the resonant frequency of below. The detected signal voltage appears the crystal 58. Due to the electronic and capaci across the condenser 93, which condenser is pos tance coupling between the upper terminal of the sessed of exceedingly low impedance at the center tuned circuit 60 and the lower terminal of the carrier frequency and exceedingly high imped crystal 58, a su?‘icient driving voltage is applied ance at audio frequencies. This voltage is im across the crystal 58 through the tuned circuit pressed across the voltage dividing network com 52 to maintain the oscillation of the crystal; prising the resistors 98 and I00 through the car This coupling also serves to maintain the tuned rier frequency decoupling resistor 94 and the au circuit 60 oscillating at its resonant frequency. dio frequency coupling condenser 91. The por In order to condition the system for signal 40 tion of this voltage which appears between the reception after signal transmission has been ef wiper 99 and ground is impressed across the input fected in the manner explained above, the push circuit of the audio frequency ampli?er 28 in an to-talk switch I38 is released. Incident to the obvious manner. The audio frequency signal restoration of this spring biased switch to its voltage as impressed across the input side of the normal position, the cathodes lSa and Ila of the 45 audio frequency ampli?er 28 is ampli?ed in this tube 83 and the tube provided in the power ampli ampli?er and transmitted to the loud speaker 29 ?er H’ are deenergized in an obvious manner. for reproduction. Thus, the transmitter mixer stage IB and the Referring now more particularly to the opera power ampli?er stage I‘! of the transmitter chan tion of the discriminator 21, it will be noted that nel it! are rendered inactive without . in any 50 this circuit is essentially a four terminal bridge way interrupting or otherwise affecting the oper circuit two arms of which respectively include the ation of the preceding stages I3, l4 and I5. In condensers 87b and 810 of equal capacitances. A this regard it is pointed out that when space third arm of the bridge comprises the capacitive current ?ow through the mixer tube 63 is inter impedance of the diode 88. l The fourth arm of rupted, the. operation of the oscillator section 55 the bridge comprises the combined capacitive im of this tube, i. e. that portion of the tube input pedance of the diode 89 and the condenser 92. circuit which comprises the intercoupled crystal The inductance element 81a is bridged between 58 and resonant circuit 60, is arrested. Thus, two terminals of the bridge circuit and the fre no carrier voltage is produced in the transmis quency modulated. signal voltage is applied to the sion channel l0 having a frequency approaching circuit across the other two terminals thereof. the intermediate frequency used at either the Since the load resistors 99 and 9| have imped ?rst or second intermediate frequency sections ances far in excess of the capacitive impedances of the receiving channel ll. Accordingly, the of the diode legs of the bridge circuit at the fre continued operation of the three stages l3, l4 quencies involved, they may be neglected in ana and I5 of the transmission channel H can in no 65 lyzing the circuit. Again, the capacitance of the way interfere with the reception of the selected condenser 93 is so much greater than that of signal modulated carrier. ' Assuming that the system is conditioned for signal reception in the manner explained above, and that the tunable stages of the system are appropriately tuned to the center frequency .of a desired frequency modulated signal carrier, the signal carrier voltage appearing across the an either diode leg-Of the circuit, that this condenser may also be neglected in analyzing the circuit. With this bridge circuit arrangement the voltage appearing at the output side of the discriminator is the difference between the absolute values of the voltages to ground at the upper and lower terminals of the inductance element 81a. From an examination of the bridge, it will be under tenna ground circuit 19 is transmitted through the tunable circuit l8 and the coupling condenser 75. stood that if the capacitance of the condenser 81b 22,468,791“ 13 equals that of the ‘condenser 810, which it does, and the capacitances'of the two diode legs of the circuit are equal, such that the bridge is balanced, the currents respectively traversing the condens 14 relativemagnitudes of the currents traversing the circuit-elements 8.‘la,l8~lb and 810 so that the volt ageb'etween the lower terminal of the inductance element 87a and ground exceeds that between the ers 81b and 810 are equal so that equal ‘voltagev Cl upper terminal of the inductance element 81av and ‘ground. As a result, an output voltage dropsappear across these condensers. Accord‘ which negative with respect to ground is pro ingly, no difference between the voltages to duced ‘between the cathode of the diode 88 and ground is developed at the upper and lower ground. It has been found that the extent or terminals of the inductance element 81a, regard-' less of the frequency of the exciting voltage ap-~ 10 magnitude of the discriminator output voltage varies in accordance with the departure of the plied to the circuit. In the actual circuit, how exciting frequency from the center intermediate ever, the capacitance of the leg which includes frequency to which the discriminator network 21 the diode '89 is greater than the capacitance of the leg including thediode-88 by an amount equal ' is center tuned. It will be understood, therefore, that if'the. frequency of the carrierappearing at to the capacitance value of the condenser 92, such the output side of the limiter 26 is frequency that the bridge is unbalanced. Accordingly dur modulated in accordance with a given audio sig-v ing excitation of the circuit, the current travers nal, a corresponding audio frequency .voltage‘is ing the condenser 81c exceeds the current trav accurately reproduced across the condenser ~93 ersing the condenser 81b so that a current is caused to flow through the inductance element 20 at the output side. of the discriminator 2.1. To consider somewhat more fully the action of 81a. ' the condenser 92in stabilizing the operation of The magnitude of this current obviously de the discriminator network 21, it may be pointed pends uponthe reactive impedance of the induct - out that if the impedances of the four legs of the ance element 81a at the particular frequency of bridge circuit are perfectly balanced, changes in excitation, and the direction of current ?ow is the exciting frequency will not produce the de such that the voltage drop across the condenser sired differences of potential between the upper 81b is enhanced and that across the condenser and lower terminals of the inductance element 810 is decreased. It will be understood, there 81a and ground. By providing the condenser v92 fore,‘ that by suitably proportioning the imped ance of the inductance element 81a relative to 30 connected in the manner illustrated, however, thereby to insure that the over-all capacitance the reactive impedance of the condensers 81b and between the lower terminal of the ‘inductance ele 8T0. at ‘a particular center frequency to establish ment. Bid and ground exceeds that between the a given relationship between the currents trav upper terminal’ of this element and ground, the ersingfthe circuit elements 81a, 81b and 8-10, the desired circulating current within the resonant absolute voltages between the upper and lower terminals of the inductance element'illa and, ground become equal. In their relationship to each other,,however, these, voltages are out of phase so that a difference voltage actually exists between the upper and lower terminals of the I circuit 81 will always be produced to insure sta bility of circuit operation. In this regard it is pointed out that the unbalancing or stabilizing condenser .92 may be connected either between the ‘lower terminal of the resonant circuit '81 and ground/or between the upper terminal of this cir cuit and ground. In either case, the desired op eration of the ‘network is produced. It is noted, however, that when a condenser 92 .of appropriate ductance element 870. to ground. The particular frequency at which these absolute voltages be 4-5 capacitance value is connected between the upper terminal of the circuit 81 and ground, the direc come ‘equal to balancethe bridge represents the tion of circulating current flow within the circuit center frequency at which the 'voltage appearing is reversed. Accordingly, the polarity of the out atjthe output side of the discriminator between put voltage produced across the condenser 93 in the cathode'of the diode .88 and ground becomes cident ‘to a given departure of the exciting fre zero. In'this regard it is pointed out that when quency from the center intermediate frequency is the bridge is balanced so that the voltages from the reverse of that which is obtained for the the upper and lower terminals of the inductance same frequency departure when the condenser element 26a to ground are equal, equal direct 92 is connected between the lowerterminal of the voltages are produced across the load resistors 99 resonant circuit and ground. and '91. These voltages'are opposingly'combined If desired, one rectifying section of the_im in a direct current path through the inductance proved discriminator 21. may be combined with element 81 so that when equal, no direct voltage‘ the audio frequency ampli?er v28 in the manner appears between the cathode of the diode 88 and illustrated in Fig. 5 of the drawings, wherein ref ground. ' ' erence characters‘ corresponding to those ,used in As the exciting vvoltage for‘the resonant circuit Fig. 2 identify the same circuit elements. From 81 is increased above the center ‘frequency, due an examination of the Fig. 5 arrangement, it will to the signal 'modulation'thereof at an audio rate, be seen that the diode section of the tube 89 is the reactive impedances of the circuit constants utilized as one of the rectifying paths of the dis change to alter the relative magnitudes of the criminator, and that the cathode, anode and currents traversing the circuit. elements 81a, Bl'b' three grids of the tube are used to amplify the and. 810, so that the voltage from the upper ter circuit 81. rI‘his difference voltage is, of course,v equal to ‘the vector sum of the absolute voltages from the ‘upper and lower terminals of the in minal of ‘the inductance element 81a to v‘ground exceeds that between the lower terminal of the inductance element ?lva'and ground." Accord ingly, a voltage which‘ is positive with respect to ground is produced between the cathode of the‘ diode 88 vand ground. If, on the other hand, the exciting frequency for the circuit 81 is vdecreased audio'frequency voltage which is developed.'be-' tween the wiper 99 and ground during reception of a selected signal. This audio voltage is trans Ymitted to the loud speaker 29 for reproduction through a coupling transformer I39. The man ner in which the audio section of the tube is blocked under the control of the muting oscilla tor '33 and mode of operation of the discriminator ances'of the ‘circuit constants change 'to alter‘the 75 21 are exactly the same as ‘explained herein ‘with below the center frequency, ‘the ‘reactive imped 15 2,408,791 16 reference to the system shown in Figs. 1 and 2. In fact, the circuit of Fig. 5 may be directly sub plied to the control grid of the modulator tube stituted for the discriminator 21 and the audio quency decoupling resistor 94, the audio frequency decoupling resistor 95 and the ?lter resistors 13! frequency ampli?er 28 in the system of Figs. 1 38 over a path which includes the radio fre and 2 to perform in the same manner, when the Cl and 31. At this point it is noted that the audio indicated connections are made between this cir frequency ?lter comprising the decoupling re cuit and the limiter 28, the high pass ?lter 38, sistor 95 and the by-pass condenser 96 prevents the transmitter mixer [8, the muting oscillator and recti?er 33, and the modulator stage I3. the'audio ‘frequency components of the voltage appearing at the output side of the discriminator 10 21 from being impressed between the input elec Automatic frequency control trodes ‘of the modulator tube 38. This ?lter also prevents audio frequency voltages developed dur As previously indicated, provisions including ing signal transmission by the microphone l2 the discriminator 21 and the modulator stage [3 from being impressed upon the input side of the of the transmission channel ID, are made in ac cordance with the present invention for automat 15 audio frequency ampli?er 28 through the coupling condenser 91. ically adjusting the output frequency of the fre As the bias applied to the tube 38 increases, the quency doubler [5 so that the difference between magnitude of the out of phase current component this frequency and the center frequency of a se traversing the space current path of the mod lected carrier is held at a substantialy constant value which substantially equals the center in 20 ulator tube 38 and the tuned circuit 4| changes so that the frequency of the voltage developed termediate frequency to which the resonant cir by the master oscillator 14 increases. As this cuits of the ?rst intermediate frequency section frequency increases, that appearing at the out of the receiving chanel II are tuned. The pur put side of the doubler l5 obviously changes in pose of this arrangement is to correct for any drift in the output frequency of the oscillator M 25 like manner to decrease the center frequency of the signal carrier traversing the ?rst intermedi or in the center frequency of the received signal carrier. In this regard, it is noted that regard less of the settings of the tuning elements pro vided in the tunable stages [8 and 28 of the re ceiving channel I I, these stages are broadly tuned to the center carrier frequency which corresponds to the settings of the tuning elements, so that ir respective of any drift in the center frequency of the received carrier all modulation components ate frequency section and increase the center frequency of the signal carrier traversing the scond intermediate section of the receiving chan nel II. The resulting increase in the center ex citation frequency of the resonant circuit 81 pro duces a corresponding decrease in the rate of in crease of the negative bias voltage applied be tween the control grid and cathode of the mod of they received signal are passed through these 35 ulator tube 38. As the bias applied to the tube 38 continues to increase at a constantly decreas ing rate the center frequencies of the signal car riers traversing the ?rst and second intermedi second intermediate frequency sections of the ate frequency sections of the receiving channel channel I I are somewhat broadly tuned in order to-permit, within limits, deviations in the center 40 ll change in like manner until the two factors become balanced. It will be understood, there carrier frequencies appearing therein without fore, that when the received carrier frequency cutting off the modulation components of the is stabilized at a particular above-normal value, frequency modulated carriers which are trans the bias voltage applied to the modulator tube mitted therethrough. It will be understood, there 38 is likewise stabilized at a particular value. fore, that by providing the improved automatic Moreover, if the circuit constants of the system frequency control arrangement described below, are properly chosen, this bias voltage will in each any drift in the output frequency of the oscillator instance be stabilized at a value such that the M or in the center frequency of a received signal center frequencies of the signal carriers travers carrier is substantially corrected in so far as the intermediate frequency sections of the receiving 50 ing the ?rst and second intermediate sections of the receiving channel II will be held at values channel and the discriminator 21 are concerned. which closely approximate the center frequencies Briefly to consider the manner in which the at which these sections of the receiving channel output frequency of the oscillator I4 is automat and the discriminator 21 are designed to operate. ically controlled, it may be assumed that the cen If the center frequency of a selected signal ter frequency of the received signal carrier starts 55 carrier drifts to a value below the center fre to drift to a value higher than the center fre quency to which the stages l8 and 20 are tuned, quency to which the resonant circuits of the or the output frequency of the oscillator Id drifts tunable stages 18 and 28 are tuned, or that while from its established value to a higher value, the this center carrier frequency remains constant, the output frequency of the oscillator I 4 starts 60 center frequencies of the signal carriers travers ing the ?rst and second intermediate frequency to drift from an established value to a lower sections of the receiving channel I I are decreased value. As the frequency drift starts and regard and increased respectively. As a result, a positive less of where it originates, the center frequency bias voltage appears at the output side of the dis of the carrier transmitted through the ?rst inter mediate frequency ampli?er 22 increases to ‘pro 65 criminator 21 which is applied to the tube 38 to duce a corresponding decrease in the center fre produce a decrease in the output frequency of the quency of the carrier transmited through the oscillator l4. The center frequencies of the signal stages of the receiving channel. In a similar manner, the ?xed tuned stages of the ?rst and second intermediate frequency stages 24, 25 and 26. ~As will be apparent from the above explana tion, this departure in the exciting frequency of the tuned circuit 8'! from the center frequency to which this circuit is tuned, causes a bias volt age, which is negative with respect to ground, to be produced between the cathode of the diode 88 and ground. This bias voltage is negatively ap carriers traversing the intermediate frequency sections ofithe receiving channel are increased and decreased accordingly. Thus, the frequency correcting action proceeds in the exact manner explained above until a point of stability is reached at which the center frequency, of the sig nal carrier voltage impressed upon the discrim 75 inator network 81 closely approximates the 2,408,791‘ 17 81 is tuned. ' Operation 01‘, the muting apparatus Referring now more particularly to the manner cutoff point. in‘ which the audio section of the receiving chan Accordingly, the voltage drops across the two resistors I18 and H9 are sharply decreased to very low values, with the result that the fullvoltage of theavailable source of anode nel I I is muted or squelched during periods when the system is conditioned for operation but is not being used either for signal transmission or re~ ception, it may be pointed out that at all times when the system is conditioned for reception but is not receiving a desired signal, noise signal volt~ ages appear in those stages of the receiver chan 18 grid of the tube II3 through the resistor H5. The negative voltage appearing across the re sistorfl In so greatly predominates over that posi tively applied to the control grid of the tube I I3 that this tube is biased beyond its space current center frequency to which the resonant circuit current is positively applied to the screen elec trode of the oscillator and recti?er tube 33. The application of this voltage to the screen electrode 10 of the tube I'Zil initiates the operation of the I oscillator section of this tube, so that an oscilla nel which precede the discriminator 21. These voltages are transmitted through the interme: ._ tory voltage is developed across the series con‘ nected coupling condenser I29 and the space diate frequency and mixer stages of the channel current path between the diode electrodes of the II and are detected by the discriminator 21 to tube. Due to the action of the diode section of the tube I20 in rectifying the oscillatory voltage, appear as audio frequency voltages at the output side of the discriminator. They may be pro duced as a result of thermal agitation within the a direct bias voltage is produced across the diode load circuit comprising the series resistors I23, I32 and 181). This bias voltage is negatively ap plied to the control grid of the ?rst tube provided tubes provided in the receiving channel, shot effects, extraneous noise voltages appearing across the. antenna-ground circuit I9, or by physical in the audio frequency ampli?er 2.8, over a path which includesithe resistors I21 and IE0 and the encircuited portion of the resistor 98. The mag nitude thereof is sufficient to bias the first audio shockto the circuit elements provided in the re ceiving channel. Regardless of the origin‘there- , o__f, however, the noise signals are manifested as audio frequency voltages across the output side of the discriminator which, in the absence of the muting apparatus provided in the system, would be passed through the audio frequency ampli?er ~23 to the loud speaker 291m repro duction. 1 frequency ampli?er tube beyond cutoif, whereby the‘no-ise signals are prevented from being trans mitted through the audio channel of the receiver to the loud speaker 29 for reproduction. As will be apparent from further considera _ tion of the curve shown in Fig. .4 of the draw More speci?cally considered, the noise response of the receiver is graphically illustrated in Fig. 4 of the drawings, wherein the noise voltage ap pearing across the condenser 93 is plotted as a function of the selected signal carrier input volt age appearing across the antenna-ground circuit L9. From a consideration of this curve, it will be noted that when no signal carrier is being re ceived, the noise voltage appearing at the output side of the discriminator 21 is’ high and that the magnitude of this voltage is sharply reduced in response to the application of a selected signal carrier to the antenna-ground circuit I9. The decrease in vthe level ofthe noise voltage which accompanies the transmission of a selected sig nal through the receiving channel Ill, is largely effected in the amplitude limiters 25 and 26,.’ To consider the action of the muting apparatus, it-is pointed out that the noise voltage appear ing between the cathode of the diode 8,8 and 35 ings, when a selected signal carrier of substan tial magnitude appears across the antenna ground circuit I9, the limiters 25 and 26 function sharply to decrease the noise voltage developed at the output side of the discriminator 21. This produces a corresponding decrease in the bias voltage developed across the load resistor III}. When the negative bias applied to the control grid of the tube I I3 is thus reduced to a low value, the current ?ow through the resistors H8 and lie the space current path of the tube H3 is ‘sharply increased to produce a corresponding increase in the voltage drops across the two videnti?ed ~resistors. As .a result, the voltage which is positively applied to the‘ screen electrode of the oscillator and recti?er tube v‘3t through the two resistors M8 and H9 is sharply decreased to a value ‘such that operation of the oscillator sec tion ofthig tube cannot continue. When the ground at the output side of the discriminator 2‘! is impressed upon the input side of the high ‘production of an oscillatory voltage across the recti?er tube I06 and appears in ampli?ed form across the coupling condenser I08 and the diode section of the tube I06 in series. Due to the recti sectionof the receiving channel is rendered oper ative to amplify the audio frequency compo space current path of the tube I23 is thus ar rested,the negative bias voltage across the recti pass ?lter .39. This ?lter acts to pass only those components of the noise voltage having fre fier load circuit resistors 128,132 and 18b is re duced to zero, permitting the normal negative quencies above the normal signal‘ reproducing bias voltage as developed across the resistor 18b band of the receiver. For example, this ?lter may to be impressed upon the control electrode of the be designed to pass frequencies above 2;)‘ kilo cycles. The noise voltage appearing across the 60 first tube provided in the audio frequency ampli ?er 28. When this ampli?er tube is thus’un output side of the ?lter 30 is impressed between blocked or biased to a normal value, the audio the input electrodes of the noise ampli?er and fying action of the diode section of the tube I06, 2. direct voltage is produced across the load re sistor III] which varies inv magnitude in accord ance with the magnitude of the noise voltage im . nents of the received signal and to transmit the same to the loud speaker .29 for reproduction, From the foregoing explanation it will be 1.111 derstood that normally, i. e. when the system is conditioned for signal reception, the noise sig pressed between the input electrodes'of the tube 70 nals appearing in the receiving channel II are utilized to completely block the audio sec I06, This direct voltage i. e. that across there tion of the receiving channel against the trans sistor no, is negatively applied to the control grid mission or noise signals to the loud speaker 29. of the direct voltage ampli?er tube ,I~I;3 through the resistor H2 in opposition to the ?xed ‘bias More speci?cally, the component circuit elements voltage normally positively applied to the control 75 of-the muting apparatus should be so chosen that 27,408,291 id 20 in the absence of a desired signal, the negative bias voltage developed at the upper terminal of the resistor 528 is approximately 20‘ volts. To this end, from 40 to 50 volts must be positively pli?er 24, and the two limiter stages 25 and 26, . applied to the screen electrode of the oscillator and recti?er tube 33 when a tube of the com mercial 1S5 type is employed in the oscillator and recti?er stage 33. Further, the component circuit elements of the muting apparatus should be such that when a selected frequency modu lated carrier is received having a magnitude ex ceeding a predetermined low value, the voltage positively applied to the screen electrode of the tube £26 is dropped to approximately 20 volts such that operation of the oscillator section of the tube I23 is arrested. In the absence of an to the input side of the discriminator 21, effec tively blocks the enumerated stages of the receiv er against the transmission of the signal modu lated carrier which is impressed upon the input side of the tunable radio ampli?er 20 through the condenser 61. More particularly, the blocking carrier voltage which appears across the output side of the ?rst'mixer 2I in the receiver, as a re sult of the stray capacitance coupling between the circuit elements of this mixer and the circuit elements associated with the crystal 58, exceeds by several times the modulated signal carrier which appears at the input side of the mixer 2I due to the coupling between the transmitting and receiving channels through the condenser oscillatory voltage between the anode and cath 61. Since the carrier voltage as derived from ode of this tube, the only negative bias voltage the crystal 58 so greatly predominates over that applied to the control grid of the ?rst tube in transmitted through the tunable radio frequency the audio frequency ampli?er 28 is that developed 20 ampli?er 20 to the input side of the mixer 2|, across the grid leak resistor 1819, which voltage those stages of the receiver which follow the is of the order of one volt. When the apparatus is designed to have the characteristics just described, the audio channel of the receiver will at all times remain blocked during periods when a selected signal is not be ing received and will be automatically unblocked when a selected signal is transmitted through the receiving channel of the system to the dis criminator 21 ‘for detection. In this regard it will be understood that since the high pass ?lter 30 will only pass frequencies outside of the nor mal signal reproducing frequency band of the re ceiving channel, the muting apparatus is not responsive to the audio frequency components of a received signal carrier and thus this apparatus is prevented from blocking the audio section of the receiving channel against the transmission of detected signal voltages to the loud speaker 29. Blocking the receiving channel during mixer 2I are effectively blocked against the transmission of the signal modulated carrier to the discriminator 21. Thus, the loud speaker 29 is prevented from reproducing the audio fre quency voltage developed through operation of the microphone I2 when the system is condi tioned for transmission. The crystal 58 also acts in conjunction with the ?xed tuned stages of the receiver to set the radiated carrier center frequency so that this fre quency cannot be changed by the discriminator 21 and is maintained at substantially the exact desired value. Thus, due to the capacitance cou pling between the circuit elements of the ?rst mixer 2i and the circuit elements of the ?xed tuned crystal 58 and resonant circuit 60, a car rier having a frequency exactly equaling the ?rst intermediate frequency is injected into the ?rst intermediate frequency section of the receiving channel' This carrier is mixed with the fre quency produced by the oscillator section of the In considering the manner in which the re second mixer tube ‘I6 to produce a carrier in the ceiving channel II is blocked against reproduc second intermediate frequency section of the re tion of the signal components of the modulated 45 ceiving channel which exactly equals the center frequency to which the resonant circuit 8‘! is carrier radiated during operation of the trans tuned. When this carrier voltage is applied to mission channel II, it is pointed out that the equipment is deliberately designed and is physi the discriminator 27, the bias voltage appearing cally so arranged that a relatively large amount at the output side of the discriminator between of stray capacitance coupling exists between the 50 the lower terminal of the resistor 95 and ground is reduced to a negligible or zero value. More circuit elements provided in the input and out over, since the crystal 58 and the crystal ‘I? which put circuits of the ?rst mixer 2|, and the elec controls the oscillator section of the second mixer trodes of the crystal 5S and the circuit con ductors connecting these electrodes with the in tube 76 are invariably ?xed to oscillate at set put electrodes of the tube 63 and the terminals frequencies, the negligible bias voltage appearing of the tuned circuit 52. More speci?cally, the at the output side of the discriminator 21 can electrodes of the crystal 58, the elements of the not be changed or altered in the slightest degree tuned circuit 60 and the circuit elements form during transmission. Accordingly, the bias ap ing the input circuit for the mixer tube ‘ID are plied to the input electrodes, of the modulator unshielded; and the control grid of the tube ‘I0 60 tube 38 through the resistors I3I and 31 is held is spaced approximately one inch from the cir~ at a ?xed negligible value during transmission cuit conductor which connects the control grid with the result that the modulator oscillator I3, of the tube 63, the lower electrode of the con I4 are in?exibly set to produce a signal modu denser 59 and the lower electrode of the crystal 58. With this arrangement and during signal 65 lated carrier voltage having a ?xed center fre~ quency. Thus, the crystal 58, acting in conjunc transmission, when the crystal 58 and the tuned tion with the stages 2|, 22, 23, 24, 25, 2'5 and 21’ circuit 60 are oscillating, a strong unmodulated of the receiving channel II, functions to stabilize carrier voltage appears at the output side of the ?rst mixer 2I, having a frequency equal to the the center frequency of the radiated signal car center frequency to which the resonant circuits 70 rier at the de?nite and ?xed value desired. This of the ?rst intermediate frequency section of center frequency value can, however, be altered the receiving channel are tuned. This strong‘ by adjustment of the adjusting element U to alter the settings of the tuning elements-provided carrier voltage as transmitted through the ?rst intermediate frequency ampli?er 22, the second in the tunable stages I4, I5, I6, I‘! and I8 of the mixer 23, the second intermediate frequency vam 75 transmission channel, but-once the desired value transmission 2,408,791 is established it is maintained by the crystal 58 in the manner just explained. a - Within the true spirit and‘ scope of the invention. I claim: ' ' ‘ ~ 7 1. In a frequency modulated radio transmit The strong carrier. injected by the'crystal 58 ting and. receivingsystem which-includes means intermediate frequency channel of: the receiving UK for ‘selectively conditioning the system for signal transmission orreception, avreceiving channel channel ll through the capacitance couplingbe» and its associated circuit elements into the ?rst including a mixer’ stage followed by» an inter mediate frequency section, a transmission chan crystal‘ 58v and the circuit elementsofi the ?rst nel-‘including a carrieriproducing oscillator 'cou mixer 2|,also serves to control the muting appa ratusl so that the audio section 28. of the receivingr 10 pledv to said mixer stage to operate as a local oscillator during signal reception, and inversely channel I l is unblocked or rendered active during acting means coupled betweenv said intermediate signal transmission. More speci?cally, this car frequency channel and said oscillator for auto rier has the same effect, in so far as the reduction matically controlling the output frequency of‘said of noise voltages at the output side of the dis oscillator to hold, substantially constant the center 15 criminator 21 is concerned,- as does the ‘applica frequency of a frequency modulated carrier trans tion of a strong signal of a selected center carrier mitted through the intermediate frequency sec frequency to the antennaground circuit I9. Ac tion of said receiving channel during signal cordingly, and as will be apparent by reconsider reception. ing the curve‘ shown in’ Fig. 4 of the drawings, tween the circuit elements ‘associated with the 2. In a combined radio transmitting and re when the strong carrier is injected'into the ?rst intermediate frequency section of the receiving channel, the noise voltage developed between the cathode of the tube 88 and ground at the output side of the discriminator 21 drops to a negligible As a result, the oscillator section of the tube I23 stops oscillating, for reasons explained above, and the negative blocking bias isremoved from the control grid of the'?rst tube in the audio ceiving system which includes means for selec tively conditioning the system for signal trans-' mission or reception, a receiving, channel in cluding a mixer stage followed by an inter mediate frequency section, a1 transmission‘ chan nel including a carrier producing oscillator cou value. . pled to said mixer stage to operate as a'local oscillator during signal reception, and inversely acting means coupled between said intermediate frequency ampli?er 28. Thus, this amplifier is frequency channel and said oscillator for auto matically controlling the output frequency- of said oscillator to hold substantially constant the carrier frequency of a signal modulated carrier transmitted through they intermediate frequency rendered operative, and may, if desired, be used to amplify and transmit to the loud‘ speaker 29 for reproduction, any side tone voltage suitably derived from the audio channel of the trans mission channel I0; section of saidreceiving channel during signal From the foregoing explanation it will be ap parent that an improved system is provided in which all available tubes and circuit elements reception. are utilized with maximum effectiveness both dur ing reception and transmission. ’ Thus, the master oscillator I4 is not only used as a carrier pro-l transmission ‘or reception, a receiving channel including a mixer stage‘ followedby an interme ducing oscillator during’ signal’ transmission, but is also coupled to the ?rst mixer stage 2 l‘ to oper ate as a local oscillator during reception. Again the modulator stage I3 is not only used'ito ireacé tively modulate the carrier output of the oscil lator 54 during transmission but, in addition, diate frequency section and a frequency discrim inator in the order named, a transmission'channel including a modulator and an oscillator controlled ’ by said modulator to produce a frequency modu functions to provide automatic frequency control in the intermediate frequency sections of the receiving channel during signal reception. The -50 use ‘of the transmitter stages l3, !4 and‘ [5 in the dual capacities mentioned may be directly at tributed .to the provision of the mixer‘ stage V5 in the transmission channel in. Thus by provid ing at this ‘stage a mixing oscillator having ‘an ' 3..In a frequency modulated radio transmit ting and receiving system which includesv means for selectively conditioning the system for signal lated carrier during signal transmission, means coupling said oscillator to said mixer stage to operate as a local oscillator during signal recep tion, and means coupling the output side of said discriminator to said modulator'to govern the output frequency of said oscilator so that-the center frequency of a frequency modulated car rier transmitted? through the intermediate fre quency section of said receiving channel during reception is held substantially constant. e output frequency equaling the center'frequency 55 signal ‘4. In a frequency modulated radio transmit of the ?rst intermediate frequency. section of the ting and receiving system which includes means receiving channel, it becomes practical to utilize for selectively conditioning the system for'signal the three stages 13, 14' and [5 of the system in, transmission or‘ reception; a receivingchannel ‘the dual capacities indicated. This is true for 60 including a mixer stage followed by an interme the reason that by providing the mixing oscillator diate frequency section and a frequency discrimi at the mixer stage I6, transmission and reception nator in the order named, a transmission channel may be effected at the same center carrier fre including an oscillator coupled to said mixer stage quency without in any way altering the tuning of any of the tunable stages of the system as it is to operate as a local oscillator during signal re 65 ception, and means controlled by said discrimi nator for automatically controlling the output selectively conditioned for signal transmission frequency of said oscillator to hold substantially constant the center frequency of a frequency modulated carrier transmitted through the in mission mixer l6 and the power ampli?er II are required to be controlled in order selectively to 70 termediate frequency section of said receiving channel during signal reception. condition the‘system for signal transmission or or reception. Moreover, by providing the de scribed novel system arrangement, only the trans reception. 5. In a combined radio transmitting and re ceiving system which is adapted to be selective ly conditioned for signal transmission or signal While one embodiment of the invention has been disclosed, it will be understood that various modi?cations may be made therein, which are‘ 75 reception, transmission and. receiving channels 23 2,408,791 24 respectively including high frequency carrier and intermediate frequency carrier sections, two like sections of said channels including frequency se lective stages which are tuned to the same reso nant frequencies, frequency converters respec tively provided in said channels at the junctions between the high and intermediate frequency sections thereof, means coupling the intermedi ate frequency section of said transmission chan nel to the frequency converter of said receiving 10 channel so that a frequency beating carrier is supplied to said last-named frequency converter from said transmission channel during signal re ception, and means for rendering the high fre quency section of said transmission channel in active when said system is conditioned for sig nal reception. of the intermediate frequency section of said transmission channel is‘ equal to the difference between the center resonant frequencies of the radio and intermediate frequency sections of said receiving channel, a coupling path for injecting a carrier voltage into said converter from said one stage of said transmission channel during signal reception, and means for rendering the radio frequency section of said transmission chan nel inactive when said system is conditioned for ignal reception. 8. In a combined radio transmitting and re~ ‘ceiving system which is adapted to be selective~ ly conditioned for signal transmissionor signal 15 reception; a transmission channel provided with a modulator, a carrier producing oscilator, a tun able frequency doubler, a tunable mixer stage 6. In a combined radio transmitting and re and a tunable radio frequency section connected ceiving system which is adapted to be selective in tandem in the order named; a receiving chan ly conditioned for signal transmission‘ or signal 20 nel provided with a tunable radio frequency sec— reception, a transmission channel provided with tion, a tunable mixer stage, an intermediate fre an audio frequency section, a tunable interme diate frequency section and a tunable radio fre quency section in the order named, a receiving quency section and a frequency discriminator con nected in tandem in the order named; means for said one stage of said transmission channel dur tion, a tunable mixer stage, an intermediate fre quency section and a frequency discriminator connected in tandem in the order named; means for utilizing the carrier voltage developed at the output side of said doubler as a local oscillator utilizing the carrier voltage developed at the out channel provided with a tunable radio frequency 25 put side of said doubler as a local oscillator volt section, an intermediate frequency section and age for frequency conversion at the mixer stage an audio frequency section in the order named, of said receiving channel during signal reception, a frequency converter provided between the radio tuning elements respectively provided in said tun and intermediate frequency sections of said re able stages, and unicontrol means for so control ceiving channel, tuning elements respectively pro 30 ling said tuning elements that regardless of the vided in said tunable stages, and unicontrol means settings of said tuning elements the center reso for controlling said tuning elements so that re nant frequencies of said radio frequency sections gardless of the settings of said tuning elements are substantially equal. the center resonant frequencies of said radio fre 9. In a combined radio transmitting and re quency sections are substantially equal and the 35 ceiving system which is adapted to be selectively center resonant frequency of at least one stage conditioned for signal transmission or signal re of the intermediate frequency section of said ception; a transmission channel provided with transmission channel is equal to the difference a. modulator, a carrier producing oscillator, a tun between the center resonant frequencies of the able frequency doubler, a tunable mixer stage radio and intermediate frequency sections of said 440 and a tunable radio frequency section connected receiving channel, and a coupling path for in in tandem in the order named; a receiving chan jecting a carrier voltage into said converter from nel provided with a tunable radio frequency sec ing signal reception. . '7. In a combined radio transmitting and receiv ing system which is adapted to be selectively conditioned for signal transmission or signal re ception, a transmission channel provided with an voltage for frequency conversion at the mixer audio frequency section, a tunable intermediate stage of said receiving channel during signal re frequency section and a tunable radio frequency 50 ception, tuning elements respectively provided in section in the order named, a receiving channel said tunable stages, unicontrol means for so con provided with a tunable radio frequency section, trolling said tuning elements that regardless of an intermediate frequency section and an audio the settings of said tuning elements the center frequency section in the order named, a fre resonant frequencies of said radio frequency sec quency converter provided between the radio and tions are substantially equal, and means cou intermediate frequency sections of said receiv pling the output side of said discriminator to said ing channel, tuning elements respectively provided modulator to govern the output frequency of said in said tunable stages, and unicontrol means for oscillator so that the carrier frequency of a sig controlling said tuning elements so that regard nal modulated carrier transmitted through the less of the settings of said tuning elements the 60 intermediate frequency section of said receiving center resonant frequencies of said radio fre channel is held substantially constant. quency sections are substantially'equal and the center resonant frequency of at least one stage HENRY MAGNUSKI.